Subzero: Add branch optimization.

src/IceCfgNode.cpp

Index: src/IceCfgNode.cpp
diff --git a/src/IceCfgNode.cpp b/src/IceCfgNode.cpp
index 4de2f577db3c4cbaedbf0939947b8d20fb08e3e1..3935aec1f7f791beff5450ca68ad65f7625796fc 100644
--- a/src/IceCfgNode.cpp
+++ b/src/IceCfgNode.cpp
@@ -457,6 +457,14 @@ void CfgNode::livenessPostprocess(LivenessMode Mode, Liveness *Liveness) {
   }
 }
 
+void CfgNode::doBranchOpt(const CfgNode *NextNode) {
+  TargetLowering *Target = Func->getTarget();
+  for (InstList::const_iterator I = Insts.begin(), E = Insts.end(); I != E;
+       ++I) {
+    Target->doBranchOpt(*I, NextNode);

[jvoung (off chromium), 2014/09/18 03:56:02]

I wonder if it makes sense to search backward and bail once the terminating
branch is found. I guess this is complicated by switch statements which can have
> 1 branch.

[Jim Stichnoth, 2014/09/18 11:45:51]

I thought about this a bit, but I think that would make too strong an assumption
in the target-neutral CfgNode class about the properties of the target-specific
lowering, namely that there is only one branch optimization opportunity per
block (though it happens to be true in the current x86 lowering).  I added a
comment about this potential opportunity though.  Also, I changed the
instruction-level doBranchOpt() methods to return bool based on whether an
optimization was actually done.

BTW, the current switch lowering still only leaves one opportunity for
optimization, namely the last branch for the default case.  The other cases
lower to conditional branches with no fallthrough target, which aren't
optimized.

+  }
+}
+
 // ======================== Dump routines ======================== //
 
 void CfgNode::emit(Cfg *Func) const {